Helicopters are equipped with an external cargo hook, mounted directly under the center of gravity of the helicopter for the purpose of transporting a load externally. This enables the helicopter to transport loads which will not fit inside the helicopter. It is also a speedy and convenient way of transporting large and cumbersome loads because it is not necessary to land the helicopter and load it and then to land to unload at a final destination. Also in operating in rough terrain there may not be a suitable landing area at certain pick-up points and thus if loads could only be transported internally in the helicopter it would be necessary to first transport the load to a suitable landing area for the helicopter.
A third advantage to carrying the load externally is that greater weights can be carried. All aircraft must be flown at weights which do not exceed their authorized gross weights and almost every helicopter has a higher authorized gross weight for external than for internal loading. This is in part because an external load can be instantly released, if necessary in an emergency and consequently it is considered acceptable to push the load carrying ability of the helicopter much closer to its limits. Also the limits of the authorized gross weights is dictated by the load carrying ability of the landing gear and since an external hook operation permits the load to hang beneath the helicopter the weight of the load is not imposed on the landing gear. Yet again, some dangerous loads pose a less potential danger to the aircraft and its occupants when carried outside, rather than inside, the aircraft.
Furthermore certain equipment is designed to perform its function only while connected to the cargo hook of a helicopter such as certain types of agricultural spray booms, water buckets and aerial ignition devices.
Present external helicopter loads must first be secured either by placing the load in a net or by otherwise attaching ropes or straps, then the ends of the ropes or the rings on the perimeter of the cargo net are attached to a strap called a lanyard. The helicopter then hovers slowly and carefully over the load while a hook-up man on the ground attaches the lanyard to the helicopter's cargo hook. The pilot then moves the helicopter slowly upward until the load is in the air. At this point the load hangs at whatever attitude is dictated by the center of gravity of its mass as the helicopter moves, wind forces are introduced on the load which responds according to its own aerodynamic characteristics. Some bulky and asymmetrical loads will rotate or swing fore and aft, or swing laterally or perform some combination of these movements which become more and more pronounced as airspeed increases thus necessitating much slower flight. At the destination point the helicopter slows down until a hover is established when the load hanging again directly beneath is lowered slowly to the ground and then disengaged by a switch on the pilot's flight control.